Document WO9404893 discloses a spectrometer with flexible filters mounted within a rotating cylinder. One or more filters may then be mounted within the circumference of a cylinder. One or more said cylinders may be stacked for further filters being made available. Mounting is accomplished by flexing a planar filter into an arcuate shape and then fitting the filter into a curved slot in the cylindrical frame. There are means disclosed for receiving an incident beam from a direction parallel to the axis of the filter cylinder. There are also means disclosed for rotating said filter cylinder.
In document WO9404893, the incident light beam enters the filter cylinder parallel to the cylinder axis and mirrors are required within the cylinder for reflecting light onto the filters, causing a construction that is not compact and does not allow straightforwardly the use of interchangeable lenses.
In fact, document WO9404893 does not disclose any specific focusing arrangements or lens assemblies in general.
These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.